A boiling water reactor core using water as a neutron moderator and a cooling medium is constituted by a number of fuel assemblies, water flowing through the fuel assemblies, control rods inserted and withdrawn between the fuel assemblies, etc. In the construction of the fuel assemblies and control rods, as shown in FIG. 8 of the patent document 1, the BWR fuel assemblies are surrounded by channel boxes.
Fuel rods containing uranium, plutonium and minor actinides and arranged in square lattice are disposed within the inner area of the fuel assemblies. In an ABWR core, the length of the fuel rod which contains uranium, plutonium and minor actinides in the vertical direction is about 3.7 m. In the present invention, this length is abbreviated as an active fuel length. When the fuel assembly is constituted by such fuel rods as having different lengths, the longest fuel rod in the fuel assembly is referred to as the active fuel length. The core is constituted by 872 fuel assemblies and 205 of cross-shaped control rods.
Since water works as both moderator for neutrons and cooling medium for control rods in the boiling water reactor, it is said that the fuel rods and water are arranged as homogeneously as possible. Thus, in PWRs now under operation, fuel rods and water are arranged almost homogeneously.
On the other hand, since BWRs are the direct cycle type, wherein steam is present in the reactor core which is two phase flow, channel boxes are disposed to prevent cross flow of steam phase in the transverse direction, and cross shaped control rods are inserted into gaps between the channel boxes. In recent high burnup fuels, in order to secure 4 to 5 of H/HM (atomic number ratio of hydrogen to heavy metal; the heavy metals include uranium, plutonium and minor actinides such as Np, Am, Cm, etc.), with an increase in enrichment, water rods are disposed in the center of the fuel assembly. This structure apparently seems to be not homogeneous; however, the above-mentioned structure was necessarily employed, because of the structure comprising the channel box and cross-shaped control rods.
The basic concept that aims at the homogeneousness is common to the PWRs, however. As a result, in the ABWR reactor cores having about 155 mm of a fuel assembly lattice pitch to secure sufficient moderating effect of two-phase flow, the two-phase flow occupies 40% or more of valuable space in the reactor core. Thus, a volume rate of the fuel material area is only 20%, the volume rate being a volume rate of fuel pellet area. Although, in view of a further reduction of spent fuels and increase in fuel economy, a great increase in a discharged burnup is strongly desired. Although there was prepossession of consistency for attaining both neutron moderating and heat removal, new design for increase in H/HM so as to improve reduction of shutdown margin and reactivity coefficient of the reactor with the high enrichment is being investigated.
On the other hand, the patent documents 2 and 3 disclose a design wherein distribution of moderating material in and outside of the channel boxes is changed; the range of H/HM was 4 to 5, however. The study for increasing fuels has not been conducted.
The conventional design has been prepossessed by structure for attaining the functions of neutron moderating and heat removal; such new designs as to separate functions of water and to increase fuel loading per unit volume without losing fuel economy and thermal margin has not been investigated.    Patent document 1: Japanese Patent Laid-open Hei 3-273189    Patent document 2: Japanese Patent Laid-open Hei 7-167988    Patent document 3: Japanese Patent Laid-open Shou 63-231298